1. Field of the Invention
The present invention relates to a quantum cryptography key distributing system and a synchronizing method used in the same.
2. Description of the Related Art
A quantum cryptography key distributing system can detect optical interception (eavesdropping) in a high probability in communication between a transmitter unit (to be referred to as “Alice” hereinafter) and a receiver unit (to be referred to as “Bob” hereinafter) based upon the uncertainty principle by Heisenberg. Therefore, whether or not a distributed key is safe can be clearly confirmed. In other words, in the quantum cryptography key distributing system, it is possible to achieve higher security by using the distributed safe key as a key of Vernam cryptography in which absolute security is proved.
Next, as an example of distribution of a shared key in a quantum cryptography key distributing system, a case that phase interference (BB84: Bennett Brassard 84) is used will now be simply described. In this example, a “quantum channel” indicates a communication channel under very weak power condition that optical power transmitted from the Alice to the Bob is lower than or equal to 1 photon/bit, whereas a “classical channel” represents a communication channel in a usual optical power region.
This shared key distribution is carried out as shown in FIGS. 1a to 1C.    (1) In the Alice (transmitter unit), phase-modulating data is generated based on random data bits “A” as original data of a cryptography key and a random data of bases (+base, X base) in the modulation, and the random data bits and random data are stored as a base data A.    (2) In the Alice, an optical pulse is phase-modulated based on the phase-modulating data, and then the phase-modulated optical pulses are transmitted to the Bob.    (3) In the Bob, the optical pulses transmitted from the Alice are phase-modulated based upon a random data of the bases (+base, X base), and then is received via an interferometer.    (4) In the Bob, an optical data bits “B” received in the Bob and the bases at this time are stored as a base data B, and then are transmitted to the Alice via a classical channel.    (5) In the Alice, the base data B transmitted from the Bob is compared with the stored base data A, and bits of the base data B which have bases are not coincident with the bases of the random data bits A are discarded.    (6) A bit number of each of the bits of the random data bit A, which have not been discarded is transmitted from the Alice to the Bob via the classical channel.    (7) In the Bob, the bits of the base data B corresponding to bit numbers except for the bit numbers transmitted from the Alice are discarded.    (8) The Alice and the Bob share the cryptography key. As described above, in order to share the cryptography key in both of the Alice and the Bob, establishment of synchronization in the unit of bits is necessary.
However, the quantum cryptography key distributing system is largely different from a conventional optical communication system in that a clock signal cannot be extracted by using a quantum channel, unlike the conventional optical communication, since the optical power of this quantum cryptography key distributing system is very weak to an extent of a single photon level.
Conventional quantum cryptography key distributing systems are described in many references. For instance, there are Japanese Laid Open Patent applications (JP-A-Heisei 08-505019 as a first conventional example, and JP-A-Showa 63-107323 as a second conventional example), “Automated “plug & play” quantum key distribution” by G. Ribordy, J. D. Gautier, O. Guinnard and H. Zbinden (ELECTRONICS LETTERS, Vol. 34, No. 22, Oct. 29, 1998, pp. 2116-2117 as a third conventional example), “Plug & Play” systems for quantum cryptography” by A. Muller, T. Herzog, B. Huttner, W. Tittel, H. Zbinden and N. Gisin (Appl. Phys. Lett. 70(7), Feb. 17, 1997, pp. 793-795 as a fourth conventional example), and “interferometry with Faraday mirrors for quantum cryptography” by H. Zbinden, J. D. Gautier, N. Gisin, B. Huttner, A. Muller and W. Tittel (ELECTRONICS LETTERS, Vol. 33, No. 7, Mar. 27, 1997, pp. 586-588 as a fifth conventional example).
Of the above conventional quantum cryptography key distributing systems, a Plug & Play type quantum cryptography system proposed by Geneva University is shown in FIG. 2, and is described in the above-described third to fourth conventional examples. The Plug & Play type quantum cryptography system can compensate fluctuations of polarization in an optical fiber transfer path. Thus, the Plug & play system is expected as a system to realize a quantum cryptography key distributing system having a higher sensitivity to polarization.
In the Plug & Play system, optical pulses are transmitted from the Bob to the Alice, and the transmitted optical pulses are modulated in the Alice. Thereafter, the polarization of the optical pulses is turned by 90 degrees by a Faraday mirror, and the optical pulses are reflected to be sent back to the Bob. In the Bob, the optical pulses reflected from the Alice are phase-modulated by a phase modulator, and then interfered. Then, the optical pulses are received by a receiver. In this case, in the Bob, it is necessary in the Bob that the optical pulses transmitted to the Alice are not modulated, and the optical pulses transmitted from the Alice are modulated.
In this system, the synchronization establishments are required in the following points:    (i) when the optical pulses transmitted from the Bob are modulated (it is necessary to follow to delay variation of the optical fiber transfer path),    (ii) when the optical pulses reflected from the Alice are modulated in the Bob (it is necessary to follow to delay variation of the optical fiber transfer path), and    (iii) when the optical pulses are received in the Bob to make the timing of application of a bias to the receiver coincident with the optical pulses (reception in ultra high sensitivity in Geiger mode).
It should be noted in (ii) that the optical pulses to be sent to the Alice is not modulated, that is, the optical pulses transmitted from the Bob to the Alice, and the optical pulses transmitted from the Alice to the Bob are not entered to the phase modulator of the Bob at a same timing.
Also, in the quantum cryptography key distributing system, the synchronization establishment needs in the unit of bits even when the base data is transmitted and received. However, the quantum cryptography key distributing system is largely different from the conventional optical communication systems in that a clock signal cannot be extracted by using a quantum channel, since the optical power in the quantum cryptography key distributing system is very weak as much as a single photon level. As a consequence, various methods for establishing bit synchronization and frame synchronization by utilizing classical channels, and methods for calibrating a system have been proposed.
In the method described in the first conventional example, as shown in FIG. 3, a calibration signal is transmitted from the Alice 700 to the Bob 800, and the Bob 800 can follow to the Alice 700. In a Plug & Play system, as described in the above (ii), only the optical pulses reflected from the Alice are phase-modulated in the Bob, but the optical pulses to be sent from the Bob to the Alice are not phase-modulated. In the method of the first conventional example, when a delay variation of the transfer path is caused so that the reception timings of the optical pulses reflected from the Alice are varied, the Bob can follow the change of the timings. However, the optical pulses to be sent out from the Bob to the Alice cannot be controlled at all. As a result, the optical pulses to be transmitted from the Bob to the Alice and the optical pulses transmitted from the Alice to the Bob may be possibly entered to the phase modulator of the Bob at the same timing, depending upon the delay amount of the transfer path. Therefore, the Plug & Play system cannot be realized.
In conjunction with the above description, a light transmission system is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 63-107323). In the light transmission system of this conventional example, a light signal is inputted from a transmission unit to a first optical path and is received by a reception unit. A reference signal is transmitted from the reception unit to a second optical path which has the same characteristics as the first optical path is located under the same environment as in the first optical path. The transmission unit turns back the reference signal. The reception unit receives the turned-back reference signal, measures change of a light signal propagation state due to change of the state of the second optical path, and corrects a change of the light signal propagated on the first optical path based on the measured result.
Also, a phase adjustment circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-154088). The phase adjustment circuit of this conventional example is provided in a master unit to synchronize the phase of a transmission data signal and the phase of a reception data signal, in a communication apparatus having the master unit and a slave unit which have a transmitting section and a receiving section. The phase adjustment circuit is composed of a reception frame generating section which generates a frame signal from the master unit to the slave unit, a phase comparison section which detects a phase difference of the frame signal which has been generated in the transmitting section in the master unit and the frame signal which has been generated in the reception frame generating section and is turned-back and returned from the slave unit, and a clock changing section in which a clock signal of the data signal received from the slave unit is synchronized based on a clock signal received from the slave unit and a clock signal in the master unit. The reception frame generating section adjusts the phase quantity based on the detection result by the phase comparison section such that the phase of the frame signal generated in the transmitting section of the master unit and the phase of the frame signal generated with the reception frame generating section and returned from the slave unit is coincident with each other.
Also, a data signal transmission and reception system is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-93233). The data signal transmission and reception system in this conventional example is composed of a receiving unit which generates a clock signal, and receives data in synchronism with the clock signal, and a transmitting unit which transmits a transmission data to the receiving unit in synchronism with the clock signal from the receiving unit. The transmitting unit turns back the clock signal to the receiving unit for a predetermined period in response to a system start, and transmits the transmission data to the receiving unit in synchronism with the clock signal. The receiving unit is composed of a phase difference detecting section which detects a phase difference between the clock signal from the transmitting unit and the clock signal generated in the receiving unit for the predetermined period, a first latch section which latches the transmission data from the transmitting unit in synchronism with the generated clock signal, a second latch section which latches the transmission data from the transmitting unit in synchronism with an inverted signal of the generated clock signal, and a selecting section selects one of an output of the first latch section and an output of the second latch section based on the detection result of the phase difference detecting section.
Also, a quantum cipher apparatus is disclosed in Japanese Laid Open Patent Application (JP-P2000-517499A). In this conventional example, communication is carried out between a first station (Alice) and a second station (Bob) by using an interference meter for quantum cryptography. At least two light pulses are transmitted through a quantum channel. In the Bob, interference of the pulses is detected. The interfering pulses (more than one) run a same arm of the interference meter and are delayed when running the above quantum channel, in anther sequence. The pulses are reflected by Faraday mirrors provided on the both edges of the above quantum channel so as for influence of the polarization to be cancelled. In this system, the alignment and the adjustment of balance are not necessary between the arms of the interference meter. Also, the station can be used in such a manner of the Plug and Play.
Also, a light time-division multiplexer is disclosed in Japanese Laid Open Patent Application (JP-P2002-236271A). In the light time-division multiplexer of this conventional example, an N-channel light branching filter inputs a light pulse string of a repetition frequency f0 and separates it into N channels (N is an integer equal to or more than 2). N light modulators input a light modulation signal in synchronism with the light pulse string and modulates an intensity or a phase of the light pulse string of a corresponding channel into a light signal sequence of a bit rate f0. An N-channel light combining unit combines the light signal sequences outputted from the light modulators. N optical waveguides connect the N-channel light branching filter and the N-channel light combining unit in different route lengths so as to give different delays the light signal sequences of the respective channels respectively. The N-channel light branching filter, the N light modulators, the N-channel light combining unit, and the N optical waveguides are provided on a substrate. The light time-division multiplexer outputs a multiple light signal sequence with the bit rate of N*f0 from the N-channel light combining unit. Each of the light modulators forms a symmetrical Mach-Zehnder-type interference section composed of a light branching filter which branches the light pulse string to two semiconductor light amplifiers which are inserted into two routes, two semiconductor light amplifiers inserted in the two routes, and a light combining unit which combines the outputs of the semiconductor light amplifiers. A modulation signal is supplied to one of the semiconductor light amplifiers via a light modulation signal light combining unit which is inserted in one of the routes. Thus, the phase-modulated light pulse string and the non-phase-modulated light pulse string are outputted from the two semiconductor light amplifiers, and an intensity-modulated light signal string for the light modulation signal is outputted from the light combining unit.
Also, a dispersion measuring method is disclosed in Japanese Laid Open Patent Application (JP-P2003-177078A). In the dispersion measuring method of this conventional example, a wavelength modulating unit modulates a light signal with a period f1 and a wavelength modulation quantity Δλ, and the wavelength-modulated light signal is subjected to a data modulation based on an input signal in a light modulating unit and outputted as a light output signal. The light output signal is converted into an electric signal by a photo-electric converting unit, and the electric signal is separated into a clock signal and an output signal by a clock extracting unit. A reference timing signal generating circuit generates a reference timing signal which is synchronized with the clock signal during a period other than a measurement period and has a phase immediately before the measurement period during the measurement period. A phase detecting unit outputs a constant phase difference during the period other than the measurement period and a phase difference which varies in accordance with the modulation by the wavelength modulating unit. A dispersion detecting unit detects a dispersion value based on the phase difference.